1. Field of the Invention
The present invention relates to batteries, and in particular, to a universal battery charging device and methodology which each include determining the state of charge of a battery and then efficiently charging the battery based on the state of charge determination.
2. Description of Related Art
Devices and methods for charging and recharging various types and sizes of batteries, such as Ni/Cd batteries, lead-acid batteries, etc., are well known in the prior art. The conventional charging methods include, for example: constant current charging, constant voltage charging, and combinations of constant current and voltage charging, among others. These methods, however, have inherent shortcomings, such as causing excessive gas production (i.e. in lead acid and Ni based batteries), excessive heat production, electrolyte decomposition (i.e. in lithium ion batteries), and overcharging (particularly when utilizing a high charging current or voltage). These prior art charging methods also yield low coulombic efficiency and low energy efficiency which results in decreased service life of a battery and potentially higher charging costs.
Another prior art method for charging a battery is trickle charging. This charging method includes charging a battery with a low power charge over a long charging period and typically does not require the use of protective devices, such as over-charging or over-temperature protection circuitry, in order to protect the battery being charged from damage that can result from improper charging. Trickle charging, however, has met with some disfavor as the need for faster charging methods have become more prevalent. For example, many devices, such as electric vehicles, portable tools, etc, have become common place and consumer demand has dictated that these items be readily available for use at a moment's notice, thereby necessitating the use of fast-charge methodologies for charging and recharging the batteries which power these devices.
Prior art fast-charge methods typically utilize relatively large charge current levels in order to charge batteries within a relatively short period of time. Large current levels, however, can cause rapid heating of a battery, and can otherwise result in low charging efficiency as well as a reduction of battery service life.
U.S. Pat. Nos. 5,680,031 and 5,179,335 disclose methods of charging a battery by controlling charging power based upon the battery's resistance-free voltage (see also, K. Kordesch et al., "Sine Wave Pulse Current Tester for Batteries," J. Electrochem Soc. Vol. 107, 480-83 (1960). As is well known, the resistance-free voltage of a battery is an effective measure of the electrochemical potential of the battery without regard to the internal resistance of the battery. The principle of the charging mechanisms disclosed in the aforementioned patents is to provide a battery with a constant charging current until the resistance-free voltage of the battery increases to a first predetermined level. Once the resistance-free voltage reaches the predetermined level, the charging current is reduced to a lower constant charging current and the resistance-free voltage in the battery is then allowed to rise to a second predetermined level. This process is repeated until the battery is charged to a desired resistance-free voltage. Although these methods provide a means for determining a battery's internal condition and for charging the battery in accordance with the measured internal conditions of the battery, these methods have not addressed the need to improve charging efficiency by increasing battery charge acceptance.
For various batteries, charge acceptance typically is high when battery state of charge (SOC) is low. As the SOC increases during battery charging, charge acceptance gradually decreases with charge acceptance typically decreasing sharply when SOC reaches approximately 50% of maximum SOC, and decreasing sharply once again when SOC reaches over approximately 85%. Therefore, when utilizing a conventional charging method, battery charging efficiency typically is low when the battery's SOC is low, with the charging efficiency increasing sharply to a relatively high value until SOC reaches over approximately 85%. As charging continues beyond approximately 85%, charging efficiency typically decreases sharply. This low charging efficiency is caused by excessive gas and heat production which can also result in decreased service life of a battery.
Additionally, U.S. Pat. Nos. 5,307,000 and 4,829,225 disclose methods of battery charging which utilize one or more high, positive current pulses and which include applying one or more negative current or depolarization pulses to the battery for short durations between successive positive current pulses. However, these methods do not address the problems associated with a changing battery SOC.
Due to charge current step-down protocol, which is well known in the prior art, excessive battery heating and overvoltage conditions may occur when charging a battery.
Therefore, there is a need to provide a method for charging batteries which includes determining a battery's SOC on a frequent basis during the charging process and which adjusts its charging algorithm according to the instantaneous charge receptivity of the battery being charged in order to promote an efficient and rapid battery charging process, and thereby facilitating optimum performance from the battery.